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Author Topic: Early August Astronomy Bulletin  (Read 847 times)

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Early August Astronomy Bulletin
« on: August 08, 2021, 08:30 »
ORIGIN OF CHICXULUB ASTEROID
SOUTHWEST RESEARCH INSTITUTE

The impactor believed to have wiped out the dinosaurs and other life forms on Earth some 66 million years ago likely came from the outer half of the main asteroid belt, a region previously thought to produce few impactors. Researchers have shown that the processes that deliver large asteroids to Earth from that region occur at least 10 times more frequently than previously thought and that the composition of these bodies match what we know of the dinosaur-killing impactor.
The team combined computer models of asteroid evolution with observations of known asteroids to investigate the frequency of so-called Chicxulub events. Over 66 million years ago, a body estimated to be 6 miles across hit in what is now Mexico’s Yucatan peninsula and formed Chicxulub crater, which is over 90 miles across. This massive blast triggered a mass extinction event that ended the reign of the dinosaurs. Over the last several decades, much has been learned about the Chicxulub event, but every advance has led to new questions. To probe the Chicxulub impact, geologists have previously examined 66-million-year-old rock samples found on land and within drill cores. The results indicate the impactor was similar to the carbonaceous chondrite class of meteorites, some of the most pristine materials in the solar system. Curiously, while carbonaceous chondrites are common among the many mile-wide bodies that approach the Earth, none today are close to the sizes needed to produce the Chicxulub impact with any kind of reasonable probability. To explain their absence, several past groups have simulated large asteroid and comet breakups in the inner solar system, looking at surges of impacts on Earth with the largest one producing Chicxulub crater. While many of these models had interesting properties, none provided a satisfying match to what we know about asteroids and comets. It seemed like we were still missing something important.

To solve this problem, the team used computer models that track how objects escape the main asteroid belt, a zone of small bodies located between the orbits of Mars and Jupiter. Over eons, thermal forces allow these objects to drift into dynamical “escape hatches” where the gravitational kicks of the planets can push them into orbits nearing Earth. Using NASA’s Pleaides Supercomputer, the team followed 130,000 model asteroids evolving in this slow, steady manner for hundreds of millions of years. Particular attention was given to asteroids located in the outer half of the asteroid belt, the part that is furthest from the Sun. To their surprise, they found that 6-mile-wide asteroids from this region strike the Earth at least 10 times more often than previously calculated. This result is intriguing not only because the outer half of the asteroid belt is home to large numbers of carbonaceous chondrite impactors, but also because the team’s simulations can, for the first time, reproduce the orbits of large asteroids on the verge of approaching Earth. This explanation for the source of the Chicxulub impactor fits in beautifully with what we already know about how asteroids evolve. Overall, the team found that 6-mile-wide asteroids hit the Earth once every 250 million years on average, a timescale that yields reasonable odds that the Chicxulub crater occurred 66 million years ago. Moreover, nearly half of impacts were from carbonaceous chondrites, a good match with what is known about the Chicxulub impactor. The work will help us better understand the nature of the Chicxulub impact, while also telling us where other large impactors from Earth’s deep past might have originated.


MARTIAN DUST STORM ENDED WINTER EARLY IN SOUTH
RAS

A dust storm that engulfed the entire Red Planet in 2018 destroyed a vortex of cold air around the Martian south pole and brought an early spring to the hemisphere. By contrast, the storm caused only minor distortions to the polar vortex in the northern hemisphere and no dramatic seasonal changes. Over two weeks at the beginning of June 2018, localised dust storms combined and spread to form an impenetrable blanket of dust that hid almost the entire planet’s surface. The global dust storm, which coincided with Mars’s equinox and lasted until mid-September, proved fatal to NASA’s solar-powered Opportunity rover. Scientists examined the effects of the event on the Martian atmosphere by combining data from a Mars Global Climate Model with observations from the European Space Agency’s ExoMars Trace Gas Orbiter and NASA’s Mars Reconnaissance Orbiter missions. It was a perfect opportunity to investigate how global dust storms impact the atmosphere at the Martian poles, which are surrounded by powerful jets of wind in winter. Since the last global storm in 2007, several new missions and instruments have arrived in Mars orbit, so the 2018 event was the most-observed to date.

Previous research has shown that high levels of dust in the atmosphere can have significant effects on polar temperatures and winds. The vortices at the winter poles also affect temperatures and the transport of air, dust, water and chemicals, so their disruption could mean substantial changes in the Martian atmosphere. The team found that the 2018 storm had profoundly different effects in each hemisphere. At the south pole, where the vortex was almost destroyed, temperatures rose and wind speeds fell dramatically. While the vortex may have already been starting to decay due to the onset of spring, the dust storm appears to have had a decisive effect in ending winter early. The northern polar vortex, by contrast, remained stable and the onset of autumn followed its usual pattern. However, the normally elliptical northern vortex was changed by the storm to become more symmetrical. The researchers link this to the high dust content in the atmosphere suppressing atmospheric waves caused by the extreme topography in the northern hemisphere, which has volcanoes over twice as tall as Mount Everest and craters as deep as terrestrial mountains. Global dust storms at equinox may enhance transport into the southern pole due to the diminished vortex, while the more robust northern vortex continues to act as an effective barrier. If this pattern for global dust storms holds over the course of the thousands of years that Mars maintains this particular axial tilt, it has implications for how dust is deposited at the north and south poles and our understanding of the planet’s climate history.


EVIDENCE OF WATER VAPOUR ON GANYMEDE
Space Telescope Science Institute (STScI)

For the first time, astronomers have uncovered evidence of water vapour in the atmosphere of Jupiter's moon Ganymede. This water vapour forms when ice from the moon's surface sublimates -- that is, turns from solid to gas. Scientists used new and archival datasets from NASA's Hubble Space Telescope to make the discovery. Previous research has offered circumstantial evidence that Ganymede, the largest moon in the solar system, contains more water than all of Earth's oceans. However, temperatures there are so cold that water on the surface is frozen solid. Ganymede's ocean would reside roughly 100 miles below the crust; therefore, the water vapour would not represent the evaporation of this ocean. In 1998, Hubble's Space Telescope Imaging Spectrograph (STIS) took the first ultraviolet (UV) images of Ganymede, which revealed in two images colourful ribbons of electrified gas called auroral bands, and provided further evidence that Ganymede has a weak magnetic field. The similarities in these UV observations were explained by the presence of molecular oxygen (O2). But some observed features did not match the expected emissions from a pure O2 atmosphere. At the same time, scientists concluded this discrepancy was likely related to higher concentrations of atomic oxygen (O). As part of a large observing program to support NASA's Juno mission in 2018, a team set out to measure the amount of atomic oxygen with Hubble. The team's analysis combined the data from two instruments: Hubble's Cosmic Origins Spectrograph (COS) in 2018 and archival images from the Space Telescope Imaging Spectrograph (STIS) from 1998 to 2010. To their surprise, and contrary to the original interpretations of the data from 1998, they discovered there was hardly any atomic oxygen in Ganymede's atmosphere. This means there must be another explanation for the apparent differences in these UV aurora images.

The team then took a closer look at the relative distribution of the aurora in the UV images. Ganymede's surface temperature varies strongly throughout the day, and around noon near the equator it may become sufficiently warm that the ice surface releases (or sublimates) some small amounts of water molecules. In fact, the perceived differences in the UV images are directly correlated with where water would be expected in the moon's atmosphere. This finding adds anticipation to ESA (European Space Agency)'s upcoming mission, JUICE, which stands for JUpiter ICy moons Explorer. JUICE is the first large-class mission in ESA's Cosmic Vision 2015-2025 program. Planned for launch in 2022 and arrival at Jupiter in 2029, it will spend at least three years making detailed observations of Jupiter and three of its largest moons, with particular emphasis on Ganymede as a planetary body and potential habitat. Ganymede was identified for detailed investigation because it provides a natural laboratory for analysis of the nature, evolution and potential habitability of icy worlds in general, the role it plays within the system of Galilean satellites, and its unique magnetic and plasma interactions with Jupiter and its environment. Right now, NASA's Juno mission is taking a close look at Ganymede and recently released new imagery of the icy moon. Juno has been studying Jupiter and its environment, also known as the Jovian system, since 2016. Understanding the Jovian system and unravelling its history, from its origin to the possible emergence of habitable environments, will provide us with a better understanding of how gas giant planets and their satellites form and evolve. In addition, new insights will hopefully be found on the habitability of Jupiter-like exoplanetary systems.


WHY IS METALLIC STAR HURTLING OUT OF MILKY WAY?
Boston University

About 2,000 light-years away from Earth, there is a star catapulting toward the edge of the Milky Way. This particular star, known as LP 40-365, is one of a unique breed of fast-moving stars -- remnant pieces of massive white dwarf stars -- that have survived in chunks after a gigantic stellar explosion. This star is moving almost two million miles an hour and speeding out of the Milky Way because it's a piece of shrapnel from a past explosion -- a cosmic event known as a supernova -- that's still being propelled forward. To have gone through partial detonation and still survive is unique, and it's only in the last few years that astronomers have started to think this kind of star could exist. Researchers analyzed data from NASA's Hubble Space Telescope and Transiting Exoplanet Survey Satellite (TESS), which surveys the sky and collects light information on stars near and far. By looking at various kinds of light data from both telescopes, the researchers and their collaborators found that LP 40-365 is not only being hurled out of the galaxy, but based on the brightness patterns in the data, is also rotating on its way out. Finding the rotation rate of a star like LP 40-365 after a supernova can lend clues into the original two-star system it came from. It's common in the universe for stars to come in close pairs, including white dwarfs, which are highly dense stars that form toward the end of a star's life. If one white dwarf gives too much mass to the other, the star being dumped on can self-destruct, resulting in a supernova. Supernovas are commonplace in the galaxy and can happen in many different ways, according to the researchers, but they are usually very hard to see. This makes it hard to know which star did the imploding and which star dumped too much mass onto its star partner. Based on LP 40-365's relatively slow rotation rate, astronomers feel more confident that it is shrapnel from the star that self-destructed after being fed too much mass by its partner, when they were once orbiting each other at high speed. Because the stars were orbiting each other so quickly and closely, the explosion slingshotted both stars, and now we only see LP 40-365.


REGIONS OF ANDROMEDA WHERE NEW STARS ARE BORN
University of British Columbia

Scientists have published a new, detailed radio image of the Andromeda galaxy -- the Milky Way's sister galaxy -- which will allow them to identify and study the regions of Andromeda where new stars are born. The study is the first to create a radio image of Andromeda at the microwave frequency of 6.6 GHz. Prior to this study, no maps capturing such a large region of the sky around the Andromeda Galaxy had ever been made in the microwave band frequencies between one GHz to 22 GHz. In this range, the galaxy's emission is very faint, making it hard to see its structure. However, it is only in this frequency range that particular features are visible, so having a map at this particular frequency is crucial to understanding which physical processes are happening inside Andromeda. In order to observe Andromeda at this frequency, the researchers required a single-dish radio telescope with a large effective area. For the study, the scientists turned to the Sardinia Radio Telescope, a 64-metre fully steerable telescope capable of operating at high radio frequencies. It took 66 hours of observation with the Sardinia Radio Telescope and consistent data analysis for the researchers to map the galaxy with high sensitivity. They were then able to estimate the rate of star formation within Andromeda, and produce a detailed map that highlighted the disk of the galaxy as the region where new stars are born. For the study, the team developed and implemented software that allowed -- among other things -- to test new algorithms to identify never-before-examined lower emission sources in the field of view around Andromeda at a frequency of 6.6 GHz. From the resulting map, researchers were able to identify a catalogue of about 100 point sources, including stars, galaxies and other objects in the background of Andromeda.


SUPERNOVA’S 'FIZZLED' GAMMA-RAY BURST
NASA/Goddard Space Flight Center

On Aug. 26, 2020, NASA's Fermi Gamma-ray Space Telescope detected a pulse of high-energy radiation that had been racing toward Earth for nearly half the present age of the Universe. Lasting only about a second, it turned out to be one for the record books -- the shortest gamma-ray burst (GRB) caused by the death of a massive star ever seen. GRBs are the most powerful events in the Universe, detectable across billions of light-years. Astronomers classify them as long or short based on whether the event lasts for more or less than two seconds. They observe long bursts in association with the demise of massive stars, while short bursts have been linked to a different scenario. The burst is named GRB 200826A, after the date it occurred. When a star much more massive than the Sun runs out of fuel, its core suddenly collapses and forms a black hole. As matter swirls toward the black hole, some of it escapes in the form of two powerful jets that rush outward at almost the speed of light in opposite directions. Astronomers only detect a GRB when one of these jets happens to point almost directly toward Earth. Each jet drills through the star, producing a pulse of gamma rays -- the highest-energy form of light -- that can last up to minutes. Following the burst, the disrupted star then rapidly expands as a supernova.

Short GRBs, on the other hand, form when pairs of compact objects -- such as neutron stars, which also form during stellar collapse -- spiral inward over billions of years and collide. Fermi observations recently helped show that, in nearby galaxies, giant flares from isolated, supermagnetized neutron stars also masquerade as short GRBs. GRB 200826A was a sharp blast of high-energy emission lasting just 0.65 second. After travelling for eons through the expanding Universe, the signal had stretched out to about one second long when it was detected by Fermi's Gamma-ray Burst Monitor. The event also appeared in instruments aboard NASA's Wind mission, which orbits a point between Earth and the Sun located about 1.5 million kilometres away, and Mars Odyssey, which has been orbiting the Red Planet since 2001. ESA's (the European Space Agency's) INTEGRAL satellite observed the blast as well.
All of these missions participate in a GRB-locating system called the InterPlanetary Network (IPN), for which the Fermi project provides all U.S. funding. Because the burst reaches each detector at slightly different times, any pair of them can be used to help narrow down where in the sky it occurred. About 17 hours after the GRB, the IPN narrowed its location to a relatively small patch of the sky in the constellation Andromeda. Using the National Science Foundation-funded Zwicky Transient Facility (ZTF) at Palomar Observatory, the team scanned the sky for changes in visible light that could be linked to the GRB's fading afterglow. Within a day of the burst, NASA's Neil Gehrels Swift Observatory discovered fading X-ray emission from this same location. A couple of days later, variable radio emission was detected by the National Radio Astronomy Observatory's Karl Jansky Very Large Array in New Mexico. The team then began observing the afterglow with a variety of ground-based facilities. Observing the faint galaxy associated with the burst using the Gran Telescopio Canarias, a 10.4-meter telescope at the Roque de los Muchachos Observatory on La Palma in Spain's Canary Islands, the team showed that its light takes 6.6 billion years to reach us. That's 48% of the Universe's current age of 13.8 billion years. But to prove this short burst came from a collapsing star, the researchers also needed to catch the emerging supernova. To conduct the search, the team was granted time on the 8.1-meter Gemini North telescope in Hawaii and the use of a sensitive instrument called the Gemini Multi-Object Spectrograph. The astronomers imaged the host galaxy in red and infrared light starting 28 days after the burst, repeating the search 45 and 80 days after the event. They detected a near-infrared source -- the supernova -- in the first set of observations that could not be seen in later ones. The researchers suspect that this burst was powered by jets that barely emerged from the star before they shut down, instead of the more typical case where long-lasting jets break out of the star and travel considerable distances from it. If the black hole had fired off weaker jets, or if the star was much larger when it began its collapse, there might not have been a GRB at all. The discovery helps resolve a long-standing puzzle. While long GRBs must be coupled to supernovae, astronomers detect far greater numbers of supernovae than they do long GRBs. This discrepancy persists even after accounting for the fact that GRB jets must tip nearly into our line of sight for astronomers to detect them at all. The researchers conclude that collapsing stars producing short GRBs must be marginal cases whose light-speed jets teeter on the brink of success or failure, a conclusion consistent with the notion that most massive stars die without producing jets and GRBs at all. More broadly, this result clearly demonstrates that a burst's duration alone does not uniquely indicate its origin.


DETECTION OF LIGHT FROM BEHIND A BLACK HOLE
Stanford University

Watching X-rays flung out into the Universe by the supermassive black hole at the centre of a galaxy 800 million light-years away, an astrophysicist noticed an intriguing pattern. He observed a series of bright flares of X-rays -- exciting, but not unprecedented -- and then, the telescopes recorded something unexpected: additional flashes of X-rays that were smaller, later and of different "colours" than the bright flares. According to theory, these luminous echoes were consistent with X-rays reflected from behind the black hole -- but even a basic understanding of black holes tells us that is a strange place for light to come from. Any light that goes into that black hole doesn't come out, so we shouldn't be able to see anything that's behind the black hole. It is another strange characteristic of the black hole, however, that makes this observation possible. The reason we can see that is because that black hole is warping space, bending light and twisting magnetic fields around itself. The strange discovery is the first direct observation of light from behind a black hole -- a scenario that was predicted by Einstein's theory of general relativity but never confirmed, until now. Fifty years ago, when astrophysicists starting speculating about how the magnetic field might behave close to a black hole, they had no idea that one day we might have the techniques to observe this directly and see Einstein's general theory of relativity in action. The original motivation behind this research was to learn more about a mysterious feature of certain black holes, called a corona. Material falling into a supermassive black hole powers the brightest continuous sources of light in the universe, and as it does so, forms a corona around the black hole. This light -- which is X-ray light -- can be analyzed to map and characterize a black hole.

The leading theory for what a corona is starts with gas sliding into the black hole where it superheats to millions of degrees. At that temperature, electrons separate from atoms, creating a magnetized plasma. Caught up in the powerful spin of the black hole, the magnetic field arcs so high above the black hole, and twirls about itself so much, that it eventually breaks altogether -- a situation so reminiscent of what happens around our own Sun that it borrowed the name "corona." This magnetic field getting tied up and then snapping close to the black hole heats everything around it and produces these high energy electrons that then go on to produce the X-rays. As researchers took a closer look to investigate the origin of the flares, they saw a series of smaller flashes. These, the researchers determined, are the same X-ray flares but reflected from the back of the disk -- a first glimpse at the far side of a black hole. The mission to characterize and understand coronas continues and will require more observation. Part of that future will be the European Space Agency's X-ray observatory, Athena (Advanced Telescope for High-ENergy Astrophysics).


LOW-COST BALLOON TELESCOPE TO RIVAL HUBBLE
Royal Astronomical Society

Durham, Toronto and Princeton Universities have teamed up with NASA and the Canadian Space Agency to build a new kind of astronomical telescope. SuperBIT flies above 99.5% of the Earth's atmosphere, carried by a helium balloon the size of a football stadium. The telescope will make its operational debut next April and when deployed should obtain high-resolution images rivalling those of the Hubble Space Telescope. Light from a distant galaxy can travel for billions of years to reach our telescopes. In the final fraction of a second, the light has to pass through the Earth's swirling, turbulent atmosphere. Our view of the Universe becomes blurred. Observatories on the ground are built at high altitude sites to overcome some of this, but until now only placing a telescope in space escapes the effect of the atmosphere. The Superpressure Balloon-borne Imaging Telescope (or SuperBIT) has a 0.5 metre diameter mirror and is carried to 40km altitude by a helium balloon with a volume of 532,000 cubic metres, about the size of a football stadium. Its final test flight in 2019 demonstrated extraordinary pointing stability, with variation of less than one thirty-six thousandth of a degree for more than an hour. This should enable a telescope to obtain images as sharp as those from the Hubble Space Telescope. Nobody has done this before, not only because it is exceedingly difficult, but also because balloons could stay aloft for only a few nights: too short for an ambitious experiment. However, NASA recently developed 'superpressure' balloons able to contain helium for months. SuperBIT is scheduled to launch on the next long duration balloon, from Wanaka, New Zealand, in April. Carried by seasonally stable winds, it will circumnavigate the Earth several times -- imaging the sky all night, then using solar panels to recharge its batteries during the day.
With a budget for construction and operation for the first telescope of £3.62 million, SuperBIT cost almost 1000 times less than a similar satellite. Not only are balloons cheaper than rocket fuel, but the ability to return the payload to Earth and relaunch it means that its design has been tweaked and improved over several test flights. Satellites must work first time, so typically have (phenomenally expensive) redundancy, and decade-old technology that had to be space-qualified by the previous mission. Modern digital cameras improve every year -- so the development team bought the cutting-edge camera for SuperBIT's latest test flight a few weeks before launch. This space telescope will continue to be upgradable, or have new instruments on every future flight. In the longer term, the Hubble Space Telescope will not be repaired again when it inevitably fails. For 20 years after that, ESA/NASA missions will enable imaging only at infrared wavelengths (like the James Webb Space Telescope due to launch this autumn), or a single optical band (like the Euclid observatory due to launch next year). By then SuperBIT will be the only facility in the world capable of high-resolution multicolour optical and ultraviolet observations. The team already has funding to design an upgrade from SuperBIT's 0.5 metre aperture telescope to 1.5 metres (the maximum carrying capacity of the balloon is a telescope with a mirror about 2 metres across). Boosting light gathering power tenfold, combined with its wider angle lens and more megapixels, will make this larger instrument even better than Hubble. The cheap cost even makes it possible to have a fleet of space telescopes offering time to astronomers around the world. The science goal for the 2022 flight is to measure the properties of dark matter particles. Although dark matter is invisible, astronomers map the way it bends rays of light, a technique known as gravitational lensing. SuperBIT will test whether dark matter slows down during collisions. No particle colliders on Earth can accelerate dark matter, but this is a key signature predicted by theories that might explain recent observations of weirdly behaving muons.


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